Radio Frequency Identification (RFID) is a wireless communication technology used for identifying, tracking and locating a physical object. RFID has applications in many different industries such as inventory control, tracking people and assets (e.g. mobile medical equipment, hospital patients and personnel), electronic toll collection, and access control. Generally, an RFID system is composed of multiple identifying tags that are attached to the objects, and readers that collect data from the tags. Often, RFID systems include a central processing unit that gathers and processes data from the readers. An RFID system may include thousands of tags in the range of a single reader at a given time, e.g. in a warehouse application.
RFID tags are generally classified as active or passive. Passive tags utilize energy received from a signal from the reader to transmit data, which limits the range over which passive RFID tags can be identified. Active tags utilize energy from a battery contained within the tag to generate and transmit the signal, providing better range and reliability. Active tags are typically more expensive than passive tags, and have a limited battery life. However, active tag RFID systems have an advantage in applications that require monitoring and data logging because the tags can remain operational even when they are out of the range of a reader.
Active tags may operate in duplex or simplex mode. Simplex mode tags, also referred to as transmit-only tags, benefit from low power consumption and simple tag design.
A major challenge in RFID systems is detection failure caused by signal collisions, which occur when a reader simultaneously receives signals from multiple tags.
Most of the work undertaken to address signal collisions has assumed that colliding signals cannot be detected and, therefore, has focused on anti-collision protocols in the Medium Access Control (MAC) layer, such as “Slotted Aloha” and “Binary Splitting”. MAC-based anti-collision protocols rely on the tags retransmitting data signals when a collision is detected, causing delayed identification and possibly faster depletion of the limited power resources within the tag. Further, MAC-based anti-collision protocols are not feasible in simplex, or transmit-only, mode tags because MAC-based anti-collision protocols require, either, that the tags themselves detect the signal collisions, or that the tags receive an indication from the reader that a collision has occurred.
Resolving the colliding signals at the physical layer in an RFID system would be beneficial because retransmission of the tag signals would not be necessary. However, resolving colliding signals at the physical layer in RFID systems poses unique challenges. For example, receivers utilized in wireless systems, in particular cellular systems, for which collision resolution at the physical layer has been developed, assume that users are assigned channel resources and therefore transmit in a synchronized fashion. This synchronization simplifies packet acquisition and the identification of collisions compared to RFID systems, particularly in the case of transmit-only RFID. Furthermore, in cellular systems, users can be assumed to transmit with the same frequency with relatively high accuracy and stability, which is not the case in RFID systems in which RFID tags utilize inexpensive circuitry for signal generation.
A method for resolving multiple, overlapping signals at the physical layer for an RFID system is desirable, particularly for active transmit-only RFID tags.